1. Field of the Invention
The present invention generally relates to a cellular wireless communication system. More particularly, the present invention relates to a method and apparatus for transmitting and receiving a downlink Synchronization CHannel (SCH).
2. Description of the Related Art
In a cellular wireless communication system, synchronization and cell search should precede between a transmitter and a receiver, for demodulation of received data and control information. Downlink synchronization and cell search are the process in which a User Equipment (UE) detects the start of a frame transmitted on a physical channel in a cell and identifies a cell-specific scrambling code applied to the physical channel. This process is referred to below as a cell search, in short. The UE performs a cell search by detecting a downlink SCH code.
FIG. 1 illustrates a known Orthogonal Frequency Division Multiplexing (OFDM) downlink frame structure and OFDM downlink SCH transmission timings in Evolved Universal Terrestrial Radio Access (EUTRA) that is the next-generation mobile communication standard of 3rd Generation Partnership Project (3GPP).
Referring to FIG. 1, a 10-ms radio frame 100 includes 10 subframes 106 each having two slots 101. In general, one slot carries 7 OFDM symbols 105. A Primary SCH (P-SCH) 103 and a Secondary SCH (S-SCH) 104 are transmitted in a particular slot 101 or 102 of each subframe 106 on the downlink.
In the EUTRA system, a UE acquires slot timing synchronization from a P-SCH in the first step of cell search by correlating a received signal with a P-SCH Scrambling Code (PSC) applied to the P-SCH and detecting a timing having a correlation peak.
In the second step of the cell search, from an S-SCH, the UE acquires frame timing synchronization and identifies a cell code group including a scrambling code specific to a cell. The second step is performed by detecting an S-SCH Scrambling Code (SSC). As illustrated in FIG. 1, the P-SCH 104 and the S-SCH 103 are transmitted in successive OFDM symbols of a slot. Hence, the UE eliminates the effect of a channel on a received S-SCH signal using the P-SCH detected in the first step as a channel estimation pilot for S-SCH detection and then carries out coherent detection for detecting the cell code group, thereby improving the performance of the S-SCH detection.
In the last step of the cell search, the UE detects a cell-specific code (i.e. cell-specific scrambling code) with a correlation peak by correlating a received pilot channel signal (or reference signal) with every cell-specific code within the cell code group. For example, one cell code group includes three codes in the EUTRA standard. A major asynchronous system, Wideband Code Division Multiple Access (WCDMA) performs a cell search procedure in a similar fashion.
By acquiring frame timing synchronization to a current cell and detecting the scrambling code specific to the current cell, the UE can demodulate data and control channels. The UE then can detect the cell IDentifier (ID) of the cell through demodulation of a Broadcasting CHannel (BCH).
The EUTRA standard defines three PSCs available to cells. One of the three PSCs is selected for application to a P-SCH for each cell for the reason that will be described with reference to FIG. 2. In FIG. 2, on the assumption of a synchronous network where frame transmission timing is synchronized among cells 200, 201 and 202 (cell #11, cell #23, and cell #32), P-SCHs and S-SCHs are transmitted at the same timing in cell #11, cell #23, and cell #32.
Referring to FIG. 2, Node Bs 207, 208 and 209 (Node B #1, Node B #2 and Node B #3) each manage three cells. Specifically, Node B #1 manages 11th, 12th and 13th cells 200, 203, and 204 (cell #11, cell #12 and cell #13), Node B #2 manages 21th, 22th and 23th cells 201 (cell #21, cell #22 and cell #23), and Node B #3 manages 31th, 32th and 33th cells 202 (cell #31, cell #32 and cell #33). A first UE 205 (UE #1) is located at the boundary among cells #11, #23 and #32, and a second UE 206 (UE #2) is located at the center of cell #23. If all cells use the same PSC, cell #11, cell #23 and cell #32 transmit PSCs 210, 212 and 214 that are identical. Cell #11, cell #23 and cell #32 use SSCs 211, 213 and 215, respectively (SSC #k, SSC #n, and SSC #m) for S-SCHs, which are the codes of cell code groups to which cell #11, cell #23 and cell #32 belong.
When UE #2 succeeds in detecting the PSC 212 from cell #23 in the first cell search step, UE #2 detects SSC #n being the code of a cell code group applied to an S-SCH in cell #23 using the PSC 212 as a channel estimation pilot. In the same manner, when UE #1 succeeds in detecting any of the PSCs 210, 212 and 214 in the first cell search step, UE #1 detects the code of a cell code group applied to an S-SCH using the detected PSC.
Since UE #1 simultaneously receives the PSCs 210, 212 and 214 from the three cells 200, 201 and 202 due to the UE #1 location, UE #1 achieves a combined channel response with respect to the cells 200, 201 and 202 by channel estimation of the P-SCH. However, SSC #k, SSC #n and SSSC #m are specific to the respective cells 200, 201 and 202. Thus, the UE #1 cannot detect an intended SSC from the combined channel response.
That's why the EUTRA standard defines three PSCs for P-SCHs. As illustrated in FIG. 3, different PSCs are typically allocated to adjacent cells so a UE at a cell boundary can use a P-SCH as a channel estimation pilot for S-SCH detection even in a synchronous network. As a first UE 305 (UE #1) is located at the boundary among 13th, 11th and 21th cells 307, 309 and 310 (cell #13, cell #11 and cell #21), UE #1 receives different PSCs 302, 303 and 304 (PSC1, PSC3 and PSC2) from the cells 307, 309 and 310. By channel estimation based on any of the PSCs 302, 303 and 304, UE #1 can achieve a channel response specific to a cell. For instance, UE #1 achieves a unique channel response between cell #13 and UE #1 from PSC1 and uses PSC1 for channel compensation and detection of an S-SCH transmitted from cell #13. In the same manner, a second UE 306 (UE #2) receives different PSCs 308, 311 and 301 (PSC1, PSC3 and PSC2) from adjacent cells and processes them.
However, the use of a plurality of PSCs for an improved cell search performance in the synchronous network degrades cell search performance in an asynchronous network. Since frame transmission timings are asynchronous among cells in the asynchronous network, even though every cell transmits the same PSC, a UE has a very low probability of receiving the PSC from cells at the same timing. When the plurality of PSCs are nevertheless used for the asynchronous network, the UE should conduct a hypothesis test against the PSCs in the first cell search step. As a result, cell search complexity is increased and PSC detection performance is degraded, as compared to the case of using a single PSC.